Google-chrome – How to make Google Chrome open PDFs in the internal PDF viewer instead of downloading them automatically

The "Redirector" answer was just what I was looking for, but that extension is no longer available. So I kept digging and found Modify Content-Type. It's working as I expected (PDFs open in the internal viewer despite Content-Disposition: attachment in the response headers) with a rule of:

URL Filter:       .*
Original Type:    application/.*pdf
Replacement Type: application/pdf
Disposition:      inline

Note well that this may expose you to cross-site scripting vulnerabilities, and you would be wise not to allow .* in the URL filter. Add to it only as needed.

I think that the problem sometimes occurs when the web server you are downloading the PDF from does not serve the PDF with the correct MIME type.

When the web server serves a PDF with the correct MIME type, Chrome previews it. When the web server serves a PDF with the incorrect or generic MIME type (application/octet-stream), Chrome downloads it.

If the server returns a Content-Type of text/html Chrome will display/open the PDF directly in the browser. However, if the server returns a Content-Type of application/pdf then the browser will prompt the user to save/download the PDF.